Twisted pair cables have long been a cost effective way to send electrical communication signals over short distances, such as inside an office building. At first, these communication signals included only the voice signals from telephones. Later, it became common to also use twisted pair to transmit the data signals associated with computer devices. Twisted pair worked well for sending data signals when only a few computer terminals needed to be directly connected to a central computer, in point-to-point fashion.
With the advent of inexpensive personal computers, and the resulting growth in the use of computers by almost every office worker, today's office building typically has computer devices in every room. To interconnect so many devices in point-to-point fashion is neither practical nor cost effective, since rarely is it necessary for every device to communicate with all other devices at the same instant.
Certain communication signalling techniques, widely known as local area networks or LANs, are becoming a popular way to interconnect large numbers of computer devices located within the same building. Using these techniques, each device can communicate with any other device at a given time, by sending LAN data signals in the form of packets over a single, shared cable.
Because many devices are sharing the single cable, each packet must be transmitted at a high speed, if the time for sending a message of a given length is to remain about the same as when the devices were connected point-to-point.
Several problems can occur if LAN signals are sent over twisted pair. First, the LAN signals have high frequency or alternating current (AC) components representative of the high speed data packets. Secondly, the LAN signals are very susceptible to corruption by external noise sources, such as nearby telephone or video equipment. This is especially true because LAN signals typically are unbalanced, that is, they also have a direct current (DC) component. The DC component is used to determine when two or more devices try to transmit at the same time. The DC component must be faithfully reproduced at to each device connected to the cable if this contention over the use of the cable is to be identified.
The use of shielded coaxial cable can prevent most corruption from external noise sources while accurately transmitting the AC components as well. Shielded coaxial cable also reduces radio emissions caused by the AC components, making it easier to meet the regulations of the Federal Communications Commission.
Conventional wisdom is that shielded coaxial cable, rather than twisted pair, should normally be used to transmit LAN signals. As a result, LAN installation usually involves wiring the building with new coaxial cables. Coaxial cable is more expensive than twisted pair cable.